1. Field of the Invention
The invention relates to a novel design of a single-phase stepper motor with a permanent magnet rotor.
2. Description of the Prior Art
A number of single-phase stepper motors of the type described above are already known which differ essentially only in the structure of the main poles and auxiliary poles. Thus, single-phase stepper motors are known wherein the pole shoes have salient main poles and auxiliary poles which project radially toward the rotor and differ by their different heights. In other known single-phase stepper motors, the two pole shoes are each provided with an approximately semicircular recess concentric with the rotor shaft. These recesses in each case serve as the main pole, while the auxiliary pole or poles are separated in space from the stator. Other known single-phase stepper motors have pole shoes with approximately semicircular recesses which are eccentric with respect to the rotor shaft. In such structures, the main poles and auxiliary poles are formed by the different distances of the recesses from the rotor periphary.
All these and other, similarly constructed, single-phase stepper motors have the disadvantage that even minor changes in position of the two stator parts with respect to each other, due to manufacturing tolerances, result in substantial increases of the starting voltage and in lowered efficiency. This is the case particularly in single-phase stepper motors intended for use in electronic battery clocks wherein, for the purpose of achieving a high efficiency and a low starting voltage, the two stator parts with the pole shoes must be mutually aligned with a precision of a few hundredth parts of 1 millimeter. To be sure, by molding centering pins that engage the stator parts to the clockwise plates, if the clockwork plates consist of metal, the required precise alignment can be accomplished, but the employment of metallic clockwork plates instead of plastic plates render the clockwork much more expensive. Moreover, even with highly precise alignment of the stator parts with repsect to each other and to the rotor, the amount of starting voltage and the efficiency are unsatisfactory in known single-phase stepper motors, which is the case especially when the motors are intended for clockworks to be operated with 1.5 volt batteries.